Substrate Requirements for Total Parenteral Nutrition

A caloric source and an amino acid source are required for adequate nutrition ( Fig. 77-21 Fig. 77-21 ). When glucose is given as the sole caloric source, 1 dL/week of 10% fat solution prevents essential fatty acid deficiency. However, a balanced daily energy supply of fat and glucose is commonly used for TPN. This approach may decrease the problems in hepatic and pulmonary function that occur with high glucose loads. ^{[169] [170] [171]}

If the total energy requirements are not met with a total dextrose intake of 4 to 5 mg/kg/min (or about 2 L of 25% dextrose for a 70-kg adult), consideration should be given to providing extra calories as fat emulsions to prevent fatty infiltration of the liver and glucose intolerance and to avoid excessive production of CO₂ . An excess of CO₂ resulting from administration of too much parenteral glucose can compromise weaning of hypermetabolic patients from mechanical ventilation or can precipitate respiratory failure in patients with severe obstructive pulmonary diseases.

The provision of 500 mL of a 20% fat emulsion has been shown to have no adverse effects on hemodynamic status or pulmonary diffusing capacity in critically ill patients.^[172] Medium-chain triglycerides may be substituted for long-chain triglycerides, with improved patient tolerance of fat infusions.^[173]

Reducing total caloric intake with TPN has reduced the incidence of complications, especially hyperglycemia. A rule of thumb is that critically ill patients require nutrition support of approximately 25 kcal/kg/day.^{[171] [174]} All solutions should contain the daily requirements of essential electrolytes. Zinc, copper, selenium, and chromium are increasingly being used to prevent abnormalities associated with trace-metal deficiencies. Daily vitamin supplementation is required.^[175] Figure 77-21A and Figure 77-21B illustrates a worksheet or order sheet for parenteral nutritional support prescription writing.